Cellular metal and method of making



United States Patent 3,268,304 CELLULAR METAL AND METHOD OF MAKINGLeonard M. Vaught, Lake Jackson, and James H. Enos,

Angleton, Tern, assignors to The Dow Chemical Company, Midland, Micln, acorporation of Delaware N0 Drawing. Filed Dec. 30, 1963, Ser. No.334,564 6 Claims. (Cl. 29-183) This invention relates to cellular metaland to a method for making the same and, more particularly, is concernedwith a novel process for preparing void-containing structures ofaluminum.

It is an object of the instant invention to provide a novel process forpreparing light weight, strong, cellular metals.

An additional object is to provide an aluminum base, open-void, cellularstructure of relatively uniform cell size whose cells may beconveniently controlled by feed stock selection.

A further object is to provide a novel process for preparing an aluminumoxide-coated aluminum cellular structure.

Still another object is to provide an aluminum nitridecoated aluminumcellular structure and a method for its preparation.

Other objects and advantages Will become apparent from reading thedetailed description of the invention disclosed hereinafter.

In general, the present invention comprises coating a metal particlewith a material which has a substantially higher melting point than themetal particle itself by heating the metal and a second material to atemperature below the melting point of the metal particle thereby toprovide a coating of the reaction product of said metal and said secondmaterial on said metal. The so-coated metal particle is subsequentlyheated to a temperature above the melting point of the metal. Thepressure on the coating of said particle is reduced While maintainingthe metal at a temperature where it is molten thereby rupturing thesurface coating on the particle and allowing at least a portion of themolten metal therein to flow out from the interior of the coatedparticle thus creating a void space within the coating, i.e., forming ahollow cell.

Cell size of the cellular structure so formed by the process of thepresent invention is dependent on the size of metal particles employed.With extremely small particles of metal, e.g., powders, the surfacetension of the molten metal may prevent its escape from the coating.With large particles, as the metal core melts, the coating itself maycollapse and no voids or cells are formed. For optimum performance,particles in the size range of from about 4 to about 50 mesh, U.S.Standard Sieve, ordinarily are used. However, particles somewhat largerand smaller than from 4 to 50 mesh, U.S. Standard Sieve cansatisfactorily be employed.

Although the resulting product ordinarily consists of a cellularstructure having open, interconnected cells and a metal matrix coatedwith the high melting surface coating, this void-containing product canbe prepared as freeflo'win-g individual cells by substantial eliminationof all the interstitial aluminum, which provides the cementing actionjoining the cells in the unitized cellular structure, from the externalsurfaces of the particle. Removal of a large portion of molten metalfrom the exterior of the ruptured cells can be readily achieved ifagitation is applied during the cell-forming stage to shake the moltenmetal down. In addition, centrifugal force may be used to help separatethe molten metal from the coated surface.

In practicing one variation of the invention, an aluminum nitride-coatedaluminum cellular structure is obtained by placing particulate aluminumhaving a particle size within the range of from about 50 to about 4mesh,

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U.S. Standard Sieve, in a vessel or reactor which permits gas flowaround and between the particles. This metal is controllably heated to atemperature above about 660 C. and below the melting point of aluminumnitride (about 2000 C.), with a preferred temperature being about 800C., in a nitrogen atmosphere at about at-mospheric pressure, employing arate of heating such that a coating of aluminum nitride forms on theparticulate aluminum prior to the metal reaching its melting point,i.e., about 660 C. When the temperature of the coated pellets reachesthe predetermined operating temperature range, the pressure on theentire charge in the furnace is reduced to an absolute pressure of fromabout 730 millimeters mercury to about 0.001 millimeter mercury. As thepressure is reduced, the aluminum nitride coating on the particlesruptures and molten aluminum flows out of the ruptured aluminum nitridecoating. A part of this molten aluminum is retained in the intersticesbetween the aluminum nitride-coated aluminum particles; the remainderflows to the bottom of the reactor vessel. The temperature of the chargeis reduced below the melting point of aluminum thereby providing analuminum nitridecoated aluminum cellular metal structure. Total shellthickness of the aluminum nitride-coated cells ordinarily ranges from0.01 inch to 0.014 inch. This cellular structure readily can beseparated from the substantially solid mass of aluminum in the bottom ofthe reactor.

Aluminum nitride is not stable in air or water and forms aluminum oxideand ammonia when exposed thereto. Therefore, as the so-formed cellularproduct is contacted with the normal atmosphere, reaction with aluminumnitride occurs thus giving a cellular product in the form of an aluminumoxide-coated aluminum cellular metal.

The rate of heating is controlled such that the reaction of aluminum andnitrogen ordinarily occurs within about 4 to about 20 minutes from thetime heat is first applied and in any event, is such that formation ofthe aluminum nitride coating substantially is complete before thetemperature reaches the melting point of the aluminum metal.

Nitrogen need be present only in amounts stoichiometrically needed toreact with aluminum to form a substantially continuous surface coatingof aluminum nitride on the particle. However, nitrogen in excess of thisamount ordinarily is used as this assures formation of the desiredcoating and produces no detrimental eiiects in the operation of theinstant invention.

In general, substantially all metals which react with a second materialto form a coating on said metal, said coating having a higher meltingpoint then said metal, can be used in the instant method.

Examples of other metals which can be used in the instant inventioninclude, for example, magnesium, tin, zinc, lead, copper and iron. Theterm metal, as used here, is meant to include both the metallic elementitself as well as alloys thereof.

Other materials which can be used to react with the metal particles ofaluminum or magnesium to form a coating on said metal particle include,for example, iammonia to form a nitride coating; oxygen or water vaporto form an oxide coating; carbon dioxide to form an oxide coating onmagnesium; sulfur-containing compounds to form sulfide coatings; andborax and other boron oxide source materials to form an oxide coating.Additional silicate-forming, sulfite-forming and sulfateformingmaterials can be used. With gaseous materials, the reactant can bepassed between the particulate coatinducing metal .as describedhereinbefore. With solid reactants, conveniently these can be blendedwith the particulate metal prior to heating to provide a mixture whereinthe metal and coating material are in close contact. With such material,the total amount of coating material is to be less than that whichreacts, on a stoichiometric basis, with all of the metal present.

The cellular structure of the instant invention finds utility as asandwich core material wherein strength and lightness are desired, forexample, for use in automobiles, aircraft, ships and the like. Inaddition, this cellular structure may be used as a metallic filter whichwould have great particle retention power due to the large void volumesin such a filter.

The following examples are merely illustrative of the invention and arein no way meant to limit it thereto.

Example I About 10 grams of particulated (-6/ +8 mesh, U.S.

tandard Sieve) aluminum was placed in a crucible which was, in turn,placed in a vacuum furnace (3 kw. unit). The furnace was filled withnitrogen. The aluminum charge was heated within a 20 minute period toabout 1100 C. range and held at that temperature for about 50 minutes.During this period, no molten metal was observed in the crucible,thereby indicating formation of an enveloping nitride coating on thesurface of each of the particles. The furnace was de-energized and thepressure reduced to about 30 millimeters mercury absolute for about a 90second period. The weight of the molten aluminum metal inside the spherealong with the decrease in external pressure burst holes in the shellwall of the particles. This molten metal flowed to the bottom of thecharge. The pressure was raised to about atmospheric and the temperaturereduced to below the melting point of aluminum. This produced a unitizedcellular structure having interconnected cells which weighed only about4 grams. About 6 grams of residual metal in the form of an apparentlysolid mass remained in the bottom of the crucible. The aluminumnitride-coated structure was separated from the residual solid residue.When contacted with water or air, this structure gave off ammoniaindicating reaction with the aluminum nitride and formation of an oxidesurface coating. The cell wall did not disintegrate which indicated thata substantial percent of the shell is aluminum metal.

Example II In order to show that reactants other than nitrogen alone maybe used in the method of this invention, a thoroughly mixed combinationof about 10 grams of aluminum powder of 6/ +8 mesh size and about 5grams of a mixture containing about 67 percent by weight H 80 about 18percent by weight B and about 15 percent by weight Na B O- were placedin a crucible which was, in turn, placed in a vacuum furnace and heatedat about atmospheric pressure from ambient temperature to about 850 C.over a period of 25 minutes in a nitrogen atmosphere. Following thisheating, a reduced pressure of about 75 millimeters mercury absolute waspulled on the vessel during a 50 second period. At these conditions, themolten aluminum broke through the nitride shell of the particles andflowed downward to the bottom of the crucible. The residual voidcontaining particles were substantially freeflowing and not cementedtogether.

About 90 percent of the resulting particles were hollow and had about a0.013 inch wall thickness. The material was not as reactive when broughtinto contact with water which indicated that the aforementioned boronoxide-containing materials had reacted with the aluminum to formaluminum oxide which became a part of the shell.

Example III About 10 grams of particulated aluminum (-8/ +20 mesh) andgrams of borax were placed in a crucible and thoroughly mixed. Thecrucible was then placed in a vacuum furnace. The furnace was filledwith argon. The charge was then heated within a 30 minute period toabout 800 C. The temperature in the furnace was then allowed to drop to750 C. over a period of 15 minutes. The furnace pressure was reduced toabout 50 mm. Hg whereupon the particles burst and molten aluminumgravity flowed to the bottom of the vessel.

When the material was cooled below the melting point of aluminum, acellular structure composed of hollow spheres of aluminum coated withsaid aluminum oxide was obtained.

In a manner similar to the foregoing examples, cellular structures ofmagnesium, tin, zinc, lead, copper and iron coated with various secondmaterials having a higher melting point than the base itself can beobtained.

Various modifications may be made in the present invention withoutdeparting from the spirit or scope thereof, and it is to be understoodthat we limit ourselve only as defined in the appended claims.

What is claimed is:

1. The method of making a cellular met-a1 structure which comprises:

(a) reacting a particulate metal with a second material at a temperaturebelow the melting point of the metal particle to provide a coating ofthe reaction product of said metal and said second material on saidmetal, said coating having a substantially higher melting point than themetal particle; V

(b) heating the so-coated particulate metal to a temperature above themelting point of said metal but below the melting point of said coating;

(c) reducing the external pressure on the coated surface of said metalthereby rupturing the coating;

(d) allowing a portion of molten metal to flow out of the coatingthrough the ruptures of said coating thus creating a void space withinthe coating; and

(e) cooling the residual void-containing structure of said metal andsaid coating material below the melting point of said metal.

2. The method in accordance with claim 1 wherein the particulate metalis aluminum and has a particle size within the range of from about 50 toabout 4 mesh, US. Standard Sieve, and the second material is nitrogen.

3. The method of making an aluminum nitride-coated aluminum cellularstructure which comprises:

(a) heating aluminum particles having a particle size within the rangeof from about 50 to about 4 mesh, US. Standard Sieve, to a temperaturebelow about 660 C. in a nitrogen atmosphere thereby coating the aluminumparticles with aluminum nitride;

(b) heating said coated aluminum particles to a temperature above about660 C. and below about about 2000 C. thereby melting the aluminum withinsaid coating;

(c) reducing the external pressure on said coated aluminum particles andrupturing the coating;

(d) allowing a portion of the molten aluminum from the interior of saidcoated particles to flow through the rupture in said coating, therebycreating a voided space within said coating; and

(e) cooling the residual void containing aluminum nitride-coatedaluminum cellular structure below 660 C.

4. The method in accordance with claim 3 and including the step ofexposing the aluminum nitride-coated aluminum cellular structure to theatmosphere.

5'. The method of making an aluminum oxide-coated aluminum cellularstructure which comprises:

(a) admixing aluminum particles having a particle size within the rangeof from about 50 to about 4 mesh, U.S. Standard Sieve, with a quantityof a boron oxide-containing material which is less than thatstoichiometrically needed to react with said (f) cooling the residualvoid containing aluminum aluminum; oxide-coated aluminum structure below660 C. (b) heating the above admixture to a temperature 6. An article ofmanufacture comprising a cellular below about 660 C. thereby coating t ealllmimetal structure composed of cells of aluminum nitridenum particleswith aluminum oxide; 5 o t d alumi (c) heating said coated aluminumparticles to a temperature above about 660 C. and below about ReferencesCited by the Examiner 2050 C. thereby melting the aluminum within saidUNITED STATES PATENTS coating; (d) reducing the pressure on said oxidecoating there- 10 2,434,775 1/1948 Sosnick 29-192 by rupturing thecoating; 2,985,411 5/1961 Madden 29192 (e) allowing a portion of themolten aluminum to 3,135,044 6/1964 M 75 2() flow through said oxidecoating through the rupture in said coating thereby creating a voidspace HYLAND BIZOT, Primary Examiner. within said coating; and 15

6. AN ARTICLE OF MANUFACTURE COMPRISING A CELLULAR METAL STRUCTURECOMPOSED OF CELLS OF ALUMINUM NITRIDECOATED ALUMINUM.